What is equivocation (the fallacy of ambiguity)?

Sliding between two meanings of the same word within an argument, so it looks valid only because a key term quietly changed meaning.

A word with more than one sense is used as if it meant the same thing throughout. The argument appears to connect, but the link depends on the shift. It fails because once you fix a single meaning, the inference collapses.

"A feather is light. What is light cannot be dark. Therefore a feather cannot be dark."

"Light" means not heavy in the first line and not dark in the second — two unrelated senses. Holding either meaning fixed breaks the chain. Equivocation is the headline case of the broader fallacy of ambiguity, where vague or shifting language props up a conclusion.

Tip: When an argument hinges on one repeated word, pin that word to a single definition and re-read — if it now falls apart, you've found an equivocation.